Carburizing is essentially the addition of carbon at the surface of low carbon steels at appropriate temperatures. Case hardening is achieved with the quenching of the high carbon surface layer that has a good fatigue and wear resistance. This layer is applied on a tough low carbon steel nucleus. Case hardening of the carburized steels is mainly a function of carbon content.
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We'd like to understand how you use our websites in order to improve them. Register your interest. Carburized steel grades are widely used in applications where high surface near hardness is required in combination with good core toughness as well as high strength and fatigue resistance.
The process of carburizing lower to medium carbon containing steel can generally provide this combination of properties and has been practiced for several decades. Such steel is essential in the vehicle power-train, machines and power generation equipment. However, the increasing performance demands by such applications as well as economical considerations forced steel producers to develop better alloys and fabricators to design more efficient manufacturing processes.
The present paper describes recent concepts for alloy design optimization of carburizing steel and demonstrates the forthcoming beneficial consequences with regard to manufacturing processes and final properties. Case carburizing steels alternatively known as case hardening steels are widely used in applications where high surface-near hardness is required in combination with good core toughness as well as high strength and fatigue resistance.
These steels have been the material of choice for several decades to manufacture components like gears, shafts or bearings. Depending on the application and the component size the following alloy systems have been established:. Chromium-nickel-molybdenum steels with high hardenability for severely loaded machinery and commercial vehicle components. Nickel-chromium steels with high hardenability for components with extraordinary toughness requirements.
Within these alloy systems, standardized steel grades are available in different major markets offering a guaranteed spectrum of mechanical properties. Case hardening steels have reached a high degree of technical maturity, which is due to the materials specification as well as a high standard in processing [ 1 ].
The manufacturing of a component involves a complex sequence of individual forming, machining and heat treatment operations including the actual case hardening treatment see Fig. During case carburizing the component is heated to a temperature in the austenite range in presence of a carbon containing gas atmosphere. During extended holding under these conditions carbon diffuses into the surface-near layer.
With the holding time increased, the diffusion depth increases. At the end of the diffusion time a concentration profile of carbon above that of the pre-existing carbon in the steel is established with the highest carbon level close to the surface. Subsequent quenching from austenite results in the formation of a hard surface layer with martensitic microstructure, especially in the carburized layer.
Deeper inside the material bainitic or ferritic-pearlitic microstructure develops due to the lower carbon content as well as the reduced cooling rate. With regard to the carburizing treatment surface hardness, case depth and core hardness are characteristic criteria that are typically specified. After carburizing and quenching, annealing at moderate temperatures can be optionally applied facilitating hard machining such as grinding.
In recent years increasing application related demands towards case carburized components indicated some shortcomings of existing alloys [ 2 ]. For instance, large gear in high-power windmills regularly showed unexpected and early failure due to gear tooth breakage or pitting damage on gear flanges [ 3 , 4 ]. Such catastrophic failure requires a complete exchange of the gearbox involving high replacement cost as well as loss of operational income due to downtime of the facility.
In the vehicle industry, several new challenges have arisen all with the aim of reducing fuel consumption and lowering emissions. Light weighting of passenger car bodies has a high priority in that respect but also weight reduction of powertrain components is increasingly being addressed. However, when a gearbox could be designed to smaller size, thus achieving lower weight, the specific operational load on the individual components will increase. Another fuel reducing trend is downsizing of the engine, which typically goes along with turbo charging.
However, the characteristic of a turbo-charged engine is an instantly much higher torque applying over the entire window of operation as compared to a naturally aspirated engine that is building up torque more gradually.
Consequently the specific load on gearbox components increases significantly with turbo-charged engines. The competitive advantage of guaranteeing a longer lifetime of the gearbox has become a driving force for material improvement. In addition, truck gearboxes can experience severely increased operational temperatures for short time periods when driving under high load with insufficient cooling [ 1 ].
The increased temperature can become high enough to cause softening of case carburized steel due to tempering effects. This softening lowers the load bearing capability of the gear and likely negatively alters the friction and wear properties at the surface. This same problem has been also observed as a cause of gearbox failure in windmills. Any possible technical improvement, however, will always be judged against its cost. Considering the typical cost structure of an automotive gear unit see Fig.
Besides those, machining also significantly contributes to the cost structure. With regard to the material cost, the contribution of alloying elements is the most relevant. While manganese and chromium are relatively cheap, molybdenum and chromium are more costly.
In general, the price of these alloying elements is not stable but subjected to volatility depending on the global supply-demand situation. There have been attempts to lower alloy cost of carburizing steels by replacing more expensive Cr-Mo or Cr-Mo-Ni steels by lower cost Cr-Mn steels. In some cases microalloying with boron has been applied to boost hardenability at comparably low cost. However, such alloy substitution always has to be checked against the service performance.
It makes no sense saving alloy cost in first place when the result is a lower performance during service and forthcoming high repair and downtime cost exceeding the initial saving. Considerable efficiency gains are possible by optimizing the carburizing heat treatment. Since carburizing is a batch process, more batches can be moved through a given furnace system per time unit, which ultimately can reduce the total number of heat treatment units required, and thus save capital investment.
It must be noted, however, that for raising the carburizing temperature to such higher level dedicated furnace equipment is needed. Furthermore, the steel subjected to the elevated temperature must resist excessive grain coarsening.
Another important cost-related aspect is due to quench distortion after carburizing. Such distortions need to be corrected by hard machining. This straightening operation requires additional processing time, rather expensive tooling and also removes part of the hard case. According to the challenges outlined above, the present paper will indicate some recently achieved improvements of case carburizing steel alloys focusing on the following targets:.
Development of an innovative alloy providing a better service performance than that of the European premium grade 18CrNiMo;. Development of a cost reduced alloy providing a similar service performance like that of the European premium grade 18CrNiMo;. This approach involves detailed knowledge of metallurgical effects of the individual alloying elements always to be considered in relation to the processing conditions during manufacturing. Some principal aspects of alloying concepts will be summarized in the following.
Hardenability of a case carburizing steel has a decisive influence on the properties related to manufacturing and machining of transmission components. High hardenability of the case carburizing steels results in more favorable shrinking behavior, leading again to a more uniform distortion during case hardening.
This makes manufacturing more predictable and reproducible. Properties such as tooth root fatigue strength and tooth flank load capacity are determined by the surface hardness, case hardening depth, and core strength. Particularly the core strength of transmission components is directly related to hardenability, which again is controlled by the alloy concept. Carbon is the most effective element with regard to hardenability see Fig.
The increased carbon content in the carburized layer by itself provides good hardenability. However, the carbon level in the base steel is limited to allow for good impact toughness. Thus other alloying elements must be added for obtaining high core hardness strength. Molybdenum, chromium and manganese are very powerful in providing increased hardenability.
Manganese is used for less demanding applications due to its comparably low cost. Additions of chromium and molybdenum to carbon-manganese base steel offer the best hardenability and are used for more demanding applications. Nickel alloying provides moderate increase in hardenability, yet the main reason for its addition is improving toughness. Higher additions of nickel can cause stabilization of retained austenite, especially in the carbon-enriched surface-near area, resulting in reduced strength and wear resistance.
For cost reduction reasons alloys using higher manganese and chromium additions, eventually combined with boron microalloying have been favored for many gear applications. However, such cost reduced alloy concepts, although providing good hardenability, have limitation in terms of toughness and tempering resistance. Besides, the prevention of intergranular oxidation requires Mn, Cr and also Si levels to be reduced. In the other extreme alloy producers have developed richly alloyed steels for those applications where transmission failure causes high replacement and outage costs.
An example is 15NiMoCr C However, such steel requires special melting technology and is not widely available. Comparing this steel to another high-Ni steel 14NiCrMo the increase of the molybdenum content from 0. Hardness loss of the carburized case after exposure 2 h to elevated temperature in low-molybdenum and high-molybdenum alloyed steels [ 9 ]. When the as-quenched microstructure after carburizing is exposed to elevated temperature, be it during service or during an additional heat treatment, the original hardness is rapidly reduced due to tempering effects see Fig.
This loss of hardness is acceptable only within strict limits, as it will reduce fatigue endurance during service otherwise. When the temperature reaches higher values, for instance under uncontrolled service conditions, the surface hardness can drop to an unacceptably low level.
In such cases steel with increased tempering resistance is required. Particularly higher molybdenum content results in a significant increase of tempering resistance and, thus, a significantly reduced hardness loss.
The carburizing treatment exposes steel to high temperature for long time. At carburizing temperature steel is in the austenitic phase allowing efficient in-diffusion of carbon from the surrounding atmosphere.
However, with increasing time and temperature, austenite grains tend to grow in size. This grain growth has negative consequences with regard to the properties of the steel after quenching. Although quenching leads to phase transformation into martensite or bainite, the prior austenite grain size PAGS is still reflected in the transformed microstructure.
Coarser PAGS results in lower yield strength, lower toughness, increased ductile-to-brittle transition temperature and larger residual stresses. Secondary negative consequences are reduced fatigue resistance and shape distortion after quenching requiring additional hard machining efforts.
Particularly detrimental in this respect is a bi-modal grain size distribution comprising smaller and larger grains together. Current industry standards therefore impose restrictions to the size and volume share of large prior austenite grains.
The metallurgical approach to avoiding excessive austenite grain coarsening during carburizing treatments is to restrict austenite grain boundary motion by dispersing small particles in the steel matrix [ 10 — 16 ].
These particles have the potential of pinning the austenite grain boundary. The size of the particles should be below nm to have grain boundary pinning potential. Furthermore, the particles should not easily dissolve at carburizing temperature.
Basics of Carburizing and Methods of Carburizing
Carburizing is one of the most widely used methods, of case hardening. The purpose of case hardening is to increase the surface hardness of steel. In this process, the steel is introduced to a carbon-rich environment and elevated to a temperature range of to o C for a certain amount of time. Due to this Carbon starts diffused into the surface layer of the material. The rate of diffusion depends upon the time and temperature zone at which the material is maintained. After carburizing material is quenched so that the carbon is locked in the structure. The hardness is moderately increased, but it can be hardened again through flame or induction hardening.
Carburizing Process and Techniques - Four Methods of Carburizing
A tough core and a hard case are the desired attributes of case-hardened steel components. This combination of properties provides wear resistance and fatigue strength at the surface, and impact strength in the core. All these components must resist wear and fatigue, have inherent toughness, and still be machinable. Typical applications include. During carburisation, the component is heated in a carbon-releasing medium to a temperature where the steel is completely austenitic. Carburization can increase the surface carbon content up to 0.
Carburising ,  carburizing chiefly American English , or carburisation is a heat treatment process in which iron or steel absorbs carbon while the metal is heated in the presence of a carbon-bearing material, such as charcoal or carbon monoxide. The intent is to make the metal harder. Depending on the amount of time and temperature, the affected area can vary in carbon content. Longer carburizing times and higher temperatures typically increase the depth of carbon diffusion.
What is carburising / carbonitriding?
We'd like to understand how you use our websites in order to improve them. Register your interest. Carburized steel grades are widely used in applications where high surface near hardness is required in combination with good core toughness as well as high strength and fatigue resistance. The process of carburizing lower to medium carbon containing steel can generally provide this combination of properties and has been practiced for several decades.